Don't print AttributeValues in EncryptionContext.toString()

https://github.com/aws/aws-dynamodb-encryption-java/blob/master/sdk1/src/main/java/com/amazonaws/services/dynamodbv2/datamodeling/encryption/EncryptionContext.java#L220

https://github.com/aws/aws-dynamodb-encryption-java/blob/master/sdk2/src/main/java/software/amazon/cryptools/dynamodbencryptionclientsdk2/encryption/EncryptionContext.java#L182

A safe `toString()` method for `EncryptionContext` would be extremely useful for logging within material providers, and we can get there by excluding attributeValues from being printed. Since it is likely that tests rely on this today, we should consider doing this with a minor bump. (Note: We haven't published the sdk2 version yet, so it's not necessary to do a bump there.)

Compare the SDK1 and SDK2 EncryptionContext.java files at the linked toString() locations, then inspect the existing tests that cover their current string output. Exclude attributeValues from the logged representation while preserving the remaining context, and consider the SDK1 minor-version bump; no SDK2 bump is needed because it has not been published.

Written by the indexing model from the issue text.